1. Field of the Invention
The present invention relates to seals used in gas turbine engines. More particularly, the present invention relates to a seal for filling a cavity formed by the forward hook region of a shroud support and a shroud. The shroud support and the shroud form a plenum region which connects to the cavity by means of a gap which lies between the forward foot regions of the shroud and shroud support. The seal of the present invention prevents cooling air in the plenum region from leaking into the cavity.
2. Discussion of the Background
In a gas turbine engine, very hot gas exits a combustor and is utilized by a turbine stage for conversion to mechanical energy. This mechanical energy drives an upstream high pressure compressor. The turbine stage is comprised of a plurality of rotor blades which are arranged circumferentially, these rotor blades are the means by which the hot gas exiting the combustor is converted to mechanical energy.
Located radially outward of the rotor blades are a plurality of shrouds which are circumferentially connected and which provide a tight radial clearance about the tips of the rotor blades so as to provide efficient operation of the engine. The shrouds are supported by a plurality of shroud supports which are located radially outward from the shrouds. Typically two shrouds are supported by each shroud support, with the shroud supports being secured to an outer casing of the engine. The shrouds and shroud supports must be segmented circumferentially to accommodate differential thermal expansion rates and to maintain blade tip clearance control.
Each shroud connects to a shroud support in such a manner that a plenum region is formed which is located between the lower mid-section of the shroud support and the upper mid-section of the shroud. This plenum region accommodates cooling air which originates from the upstream high pressure compressor. This cooling air travels in an aft direction radially outward of the combustor and makes its way to the shroud supports. Each shroud support is provided with a plurality of holes which serve as channels for directing the cooling air to the plenum region formed by the shroud and shroud support.
This cooling air in the plenum region impinges on the shrouds for purposes of cooling the shroud. This cooling is necessary because the hot gases which enter the turbine first stage are at temperatures levels which can exceed the melting point of the shrouds. Since the shrouds are exposed to such temperature levels, the cooling air is necessary for the safe operation of the engine. Each shroud is further provided with film holes which extend in a radially inward direction through the shroud for purposes of convectively cooling the shroud interior.
To the front of the plenum region is a forward foot section of the shroud support which forms a forward gap with a forward foot section of a connected shroud. The forward gap leads to a cavity area formed by a forward hook region of the shroud support and connected shroud. To the aft of the plenum region is an aft foot section of the shroud support which forms an aft gap with an aft foot section of the connected shroud.
These gaps to the front and to the rear of the plenum region have resulted in significant amounts of cooling air being wasted, the cooling air leaking out due to the fact that the air in the plenum region is under greater pressure than regions to the front and rear of the plenum region. The gaps are necessary for purposes of assembling the shroud and shroud supports as well as for purposes of accommodating thermal expansion, as has been previously mentioned.
The circumferentially segmented shrouds and shroud supports result in the formation of a forward annular cavity, a forward annular gap, an annular plenum region, and an aft annular gap.
The more air that is utilized by the engine for cooling results in a diminished amount of air which can be used for the purpose of generating thrust. Until recently the leakage paths (i.e., gaps) provided to the front and rear of the plenum region were considered to be an insignificant problem. However, the significance of the problem has been reconsidered in light of increased performance demands and higher shroud cooling requirements due to elevated gas path temperatures.
The related case entitled "Shroud Support Rail and Method for Assembling in Turbine Engine", U.S. patent application Ser. No. 07/750,993, filed concurrently herewith, has addressed the problem of leakage to the aft of the plenum region.
Therefore, a need exists for a mechanism which significantly reduces or eliminates the leakage of cooling air from the plenum region through a cavity in the forward hook region of a shroud support to the gas path, thereby bypassing the film holes.